With respect to baked products of melamine and phosphoric acid, melamine orthophosphate, melamine pyrophosphate and melamine polyphosphate have, for example, been known, and there have been many reports on processes for their production. For example, JP-B-40-28594 discloses a process for producing baked melamine phosphate having part of melamine orthophosphate remained, which comprises baking melamine orthophosphate at a temperature of from 180 to 250.degree. C. However, it has been pointed out that the melamine phosphate is a complex of melamine orthophosphate and melamine pyrophosphate, and has no adequate water resistance.
U.S. Pat. No. 3,920,796 discloses a formation of melamine pyrophosphate by baking melamine orthophosphate at a temperature of from 170 to 325.degree. C.
Further, U.S. Pat. No. 4,950,757 discloses a process for producing melamine pyrophosphate by reacting pyrophosphoric acid with melamine in an aqueous medium at a temperature of from 0 to 60.degree. C.
JP-A-61-126091 discloses a process for producing melamine condensed phosphate which comprises reacting condensed phosphoric acid and melamine in a solid phase substantially in the absence of an aqueous medium under a temperature condition of from naturally generated heat temperature to 170.degree. C.
It is known to employ urea as a condensation agent in the production of a polyphosphate. For example, JP-B-53-2170 discloses a process for producing a polyphosphoric acid amide (amide polyphosphate) containing amide-type nitrogen, which is obtainable by using ammonium orthophosphate, orthophosphoric acid, condensed phosphoric acid, phosphoric anhydride, urea phosphate, or a mixture thereof, as a phosphoric acid source, and a cyanamide compound such as melamine, dicyan cyanamide, guanidine or guanyl urea, as a nitrogen source, and subjecting a mixture thereof to a heat condensation reaction in the presence of urea, urea phosphate or a mixture thereof, as a condensation agent. As the production conditions, it is disclosed to carry out the heat condensation in a ratio of urea/phosphoric acid (as H.sub.3 PO.sub.4)/cyanamide compound=0.8-1.5/1/0.05-1 (molar ratio) in an ammonia gas atmosphere at a temperature of from 150 to 350.degree. C. for from 10 minutes to 5 hours, preferably from 1 to 4 hours.
A.C.S. Symposium Series No. 425 "Fire and Polymers", chapter 15, p. 211-238, American Chemical Society, Washington, D.C., 1990, discloses that melam ultraphosphate is formed by heating melamine phosphate [melamine/phosphorus atom=1/1 (molar ratio)] at a temperature of from 330 to 410.degree. C.
Many proposals have been made up to present with respect to utilization of melamine phosphate as a flame retardant. For example, JP-A-53-49054 discloses a polyamide resin composition having the flame retardancy improved by adding an inorganic filler and melamine phosphate to a polyamide.
JP-A-61-126091 discloses that melamine condensed phosphate is effective as a flame retardant for a thermoplastic resin such as a polyester, a polyamide or a polyolefin, a thermosetting resin such as phenol or urethane epoxy, or a cellulose material.
The melamine polyphosphate or the melamine condensed phosphate as disclosed in the above-mentioned prior art can hardly be used effectively as a flame retardant for a resin having a high molding temperature, since desorption of melamine is substantial when heated at a temperature of at least 300.degree. C.
The present invention is to overcome the drawbacks of the above-mentioned prior art and to present a melamine.melam.melem double salt of a polyphosphoric acid, which is useful for a wide range of applications as a flame retardant and which is produced from melamine and phosphoric acid as starting materials, and a process for its production.